This invention relates to the superconducting rotor of an electric motor and more particularly to cooling such a rotor to maintain its field coils in the superconducting state.
Superconducting rotating machines such as motors and generators must be cooled such that the field structures of their rotors are in the superconducting state. The conventional approach to cooling rotor field coils is to immerse the rotor in a cryogenic liquid. For example, a rotor employing conventional, low temperature superconducting materials must be immersed in liquid helium. Similarly, rotors employing field coils made of high temperature superconducting materials might be immersed in liquid nitrogen. In either case, heat generated by or conducted into the rotor is absorbed by the cryogenic liquid which undergoes a phase change to the gaseous state. Consequently, the cryogenic liquid must be replenished on a continuing basis. This replenishment is accomplished through a transfer line that feeds the cryogenic liquid into the rotating machine along the axis of the rotor. A rotating, cryogenic leak-tight seal is required in the transfer line. Rotating seals require surfaces which slide on one another in extremely close contact to prevent leakages. Since the surfaces rub, friction will wear them away and eventually create a gap. At room temperature, elastomers are used to eliminate this problem but there are no known materials which have suitable elastomeric properties at cryogenic temperatures. Consequently, rotating cryogenic leak-tight seals are available only as custom products and require frequent maintenance and parts replacement.
The dynamic stability of rotating systems employing a free liquid is difficult to maintain. Rotation can cause wave action resulting in a mechanical imbalance in the rotor. Further, because the liquid is in a rotating system, the rotational acceleration pressurizes the cryogenic fluid with the maximum pressure at the periphery of the machine. This pressurization causes the boiling point of the cryogenic liquid to be elevated. At atmospheric pressure, liquid nitrogen boils at 77 K. However, for a rotor 36 centimeters in diameter rotating at 3600 rpm, the boiling point is approximately 97 K., which is very close to or higher than the transition temperature of some ceramic oxide high temperature superconductors. Moreover, for such high temperature superconductors it is known that their performance is greatly enhanced at temperatures below the transition temperature. In the bismuth-strontium-calcium-copper oxide (BSCCO) 2223 system, for example, a three times higher magnetic field can be generated by cooling the superconductor to 50 K. as compared to the 77 K. of liquid nitrogen at atmospheric pressure.
Another approach for achieving cryogenic temperatures, though heretofore not in a rotating environment, is the cryogenic refrigerator or cryocooler. Cryocoolers are mechanical devices operating in one of several thermodynamic cycles such as the Gifford-McMahon cycle and the Stirling cycle. Cryocoolers have found application, for example, in cooling the stationary magnets in magnetic resonance imaging systems. See, for example, M. T. G van der Laan et al., "A 12 k superconducting Magnet System, Cooled via Thermal Conduction by Means of Cryocoolers", Advances in Cryogenic Engineering, Volume 37, Part B, (Proceedings of the 1991 Cryogenic Engineering Conference) edited by R. I/V. Fast, page 1517 and G. Walker et al., "Cryocoolers for the New High-Temperature Superconductors," Journal of Superconductivity, Vol. 1 No. 2, 1988. It is well known to those skilled in the art that good cryocooler performance depends on a design optimized for the actual conditions the cryocooler operates under. Known cryocoolers are not adapted for operation in a rotating environment because they usually do not have a rotational axis of symmetry or a fluid piston/regenerator designed to operate in a rotating environment.